1. Field of the Invention
The present invention relates generally to bending presses generally referred to as press brakes used to bend sheet-like workpieces such as sheet metals. More particularly the present invention pertains to methods and apparatuses for accurately and easily bending sheet-like workpieces into various shapes including those channel-like and semicircular in cross section.
2. Description of the Prior Art
As is well-known, bending presses or press brakes for bending sheet-like workpieces such as sheet metals comprise a pair of long bar-like upper and lower tools or dies which are horizontally disposed in vertical alignment with each other. The upper tool is formed at its lower surface with a horizontally extending bending portion which is generally V-shaped in cross section but is often formed otherwise. The lower tool is formed at its top surface with a horizontal groove which is also generally V-shaped in cross section but is often formed otherwise. Also, either of the upper and lower tools is horizontally fixed to a fixed beam member, and the other is horizontally carried by a movable beam-like ram member which is power driven vertically by a suitable means such as a hydraulic motor. Of course, the bending portion of the upper tool and the groove of the lower tool are in vertical alignment with each other so that they may be brought into and out of engagement with each other when the ram member is vertically moved. Thus, in operation, a workpiece to be bent is horizontally placed on the lower tool and then the ram member is moved by power to make the bending portion of the upper tool contact the workpiece and then press the workpiece into the groove of the lower tool.
In the above described arrangement, the workpiece is bent into the shape of the groove of the lower tool if it is fully pressed thereinto by the bending portion of the upper tool which has been formed similar to the groove of the lower tool. However, in what is called air bending by use of the upper tool which is V-shaped, the workpiece is bent to various angles depending upon the depths by which the bending portion of the upper tool is brought into the groove of the lower tool. In other words, the workpiece can be bent to any angle by the air bending without changing the upper and lower tools by adjusting the stroke length of the ram member to adjust the entry of the bending portion of the upper tool into the groove of the lower tool, namely, by the pressure of the upper tool onto the workpiece. Of course, the workpiece can be bent several times by the air bending to be formed into various shapes having several folds of various angles by repeatedly stroking the ram member with the stroke length adjusted. Also, it could be possible in air bending to bend the workpiece into a semicylindrical shape which is semicircular in cross section by repeatedly stroking the ram member with the stroke length adjusted and forwardly moving the workpiece by a slightest equal distance after each stroke of the ram member.
In bending presses, it is very important to accurately adjust and set the stroke length of the ram member in order to accurately bend the workpiece to desired angles as has been described above, since in fact a slight error in adjusting the stroke length of the ram member will result in poor bending. Also, it is essentially necessary to adjust and set the stroke length of the ram member not only according to the bending angle to be made on the workpiece but also based upon other conditions such as the width and configuration of the groove of the lower tool, the thickness, width and tensile strength of the workpiece to be bent. Furthermore, it is likewise necessary in adjusting and setting the stroke length of the ram member to take into account the deflections of the ram member which will inevitably occur because of the bending force during bending operations and will produce an influence on the stroke length of the ram member.
Heretofore, however, there has been neither method nor means for accurately adjusting and setting the stroke length of the ram member in bending presses. Above all things, it has been conventionally virtually impossible to accurately find out or measure the thickness and the tensile strength of workpieces to be bent, since the workpieces are delicately different in thickness and tensile strength even if they have been produced as an identical lot. Also, there has been no means for effectively detecting the deflections of the bending presses and compensating the stroke length of the ram member for such deflections. Such being the case, heretofore, it has been customary that the stroke length of the ram member is adjusted and set by trial and error by experimentally bending the workpieces until an acceptable bend is obtained. Accordingly, a great deal of skill has been required to adjust and set the stroke length of the ram member and the fact has been that a number of workpieces will be scrapped before an acceptable stroke length is obtained. Nevertheless, it has been impossible in any event to make really accurate bending operations by adjustng and setting the stroke length of the ram member in the conventional manner, since the workpieces are different in thickness and tensile strength and will produce changes in the bending force.
As another major conventional disadvantage with regard to bending presses, it has been impossible to easily and accurately bend workpieces by use of a single pair of upper and lower tools into cylindrical shapes which are semicircular in cross section. In order to bend a workpiece into a cylindrical shape by use of a single pair of upper and lower tools, it is necessary to repeatedly stroke the ram member with the stroke length adjusted at each stroke and forwardly move the workpiece by a slightest equal distance after each stroke. However, it has been virtually impossible to accurately adjust and set the stroke length of the ram member as has been described hereinbefore, and furthermore it has been impossible to accurately set the distance by which a workpiece to be bent is forwardly moved after each stroke of the ram member so that the workpiece is bent into a semicylindrical shape.